Field of the invention
The invention relates to a mask type color television tube and its method of manufacturing the same. It concerns more particularly a color television tube in which the three electron guns are aligned, i.e. the axes of said guns are positioned in one common plane.
A color television tube comprises a frontal panel upon the internal face of which is deposited the screen which is usually formed of vertical bands of cathodoluminescent substances (phosphors) emitting, when they are excited by an electron beam produced by an electron gun, a red, green or blue color light. The screen thus comprises a succession of assemblies comprising three vertical bands, each assembly presenting a red band, a green band and a blue band. Each color is excited by a corresponding electron beam. In one particular type of tube, often known as a "matrix", two adjacent bands of phosphors are separated by a black graphite band, thereby allowing to obtain an improved contrast image. In a mask tube, with a view to selecting the colors, in such a manner that the beam associated to one color, for example, blue, only strikes the phosphor adapted to produce this color (blue), a perforated mask is provided in front of the screen of which the position and the disposition of the openings, generally elongated slots extending in the vertical direction ensure the said color selection.
Since the position of the mask with respect to the screen must be accurately determined, the mask is used in order to form the screen and, with this purpose, it is secured to the panel of the tube prior to the formation of the screen. Each of the luminescent substances is thus deposited in the following manner: the internal face of the screen is coated with a solution of the substance in a photosensitive material that hardens when it is illuminated by an ultra-violet radiation (UV), then this solution is illuminated by an optical system that comprises an UV radiation source and an objective simulating the tube deflector. The position of the optical system, especially of the UV lamp, depends upon the color of the phosphor in solution. In this manner, only the photosensitive material present at the sites provided for the determined color is illuminated and can thus harden. The material present in the other sites does not harden and does not adhere to the glass; it can be cleaned by washing with water or another convenient liquid.
The mask being provided with slots disposed in succession along vertical lines, it is necessary, in order to form on the screen continuous vertical lines, to displace the UV source in the vertical direction during illumination.
It has been noted that the phosphor bands, or graphite bands, adjacent to the corners of the rectangular screen have an irregular appearance, thereby impairing the quality of the image in these zones.
This deficiency results from the deformations due to the optical system used for projecting the UV light, these deformations increasing with the distance between each dot of the screen and the axis of the tube and with the distance between each of these dots and the horizontal median plane. In particular, in the vicinity of a vertical edge of the screen, the image of a vertical slot of the mask is, on the horizontal median line, a spot that is also vertical; but, in the vicinity of the edges the image of a vertical slot of the mask is an inclined spot. Therefore, after the slight displacement of the UV source with respect to the optical projection system, close to the vertical edges of the screen, irregular phosphor or graphite lines or bands will appear, that are wider towards the corners than at the center.
In order to overcome this deficiency, a cylindrical lens is usually provided in the optical system for projecting the UV light. However, such a cylindrical lens is a costly element and, furthermore, does not give entirely satisfactory results.